Abstract: In the river system,the environmental change always undergoes the process from a quantitative change to a qualitative change.The limit of the qualitative change is known as the threshold.The threshold is the upper limit of the cumulation process.When the process reaches or goes beyond the limit, the original event series will be replaced by the other event series.The investigation shows that there is the process from a quantitative change to a qualititative change in the evolution of the fluvial fan and delta of the Huanghe River.The geometric forms formed by each process are roughly same.This threshold of the geometric forms not only proved the quantitative index for plotting the periodicity of fluvial fan and delta,but also is very important for estimating the trend of natural ervironmental change. It is shown that the thresholds of geometric forms of the third periodic fluvial fan of the Huanghe River since the middle Holocene range from 0.93 to 0.94,while the one of the fourth periodic delta since 1855 A.D.from 1.2 to 1.21. According to the change trend during the historical time and the conditions of present natural environment,the lower reaches of the Huanghe River will be able to burst its banks at the Dongbatou-Gaocun to flow northward. The controlling measure should be taken.